In fact, a word should be defined as a 'sign' that has a form (written/spoken), a signification/meaning, and is usable in some (one or more) contexts.

I'm grossly translating my linguistics notions from French, but that's about it. I like the way banj put it. An acronym isn't a word, it's multiple words put together, it doesn't really have a meaning of its own but has the meaning of the words it is composed of. I don't trust dictionaries a lot because they can't keep up with all new words or new uses of actual words. But I think English dictionaries are better than French ones for this because English accepts much more new words in its lexicon all the time, it's a language that evolves faster, it's more 'up to date'. I, and the majority of my fellow linguists, consider that if a word (as previously defined) is used and understood by a majority of people within a society, it should be considered as a valid and "true" word, be it in the dictionary or not. Those books are more annoying than anything